1. Field of the Invention
The present invention relates to an ultrasonic motor having a moving body which is driven by progressive oscillations and, more particularly, to drive control of an ultrasonic motor having a moving body which is driven by progressive oscillations.
2. Description of the Related Art
Japanese Patent Publication No. Sho 59-156168 discloses a conventional device for performing drive control of an ultrasonic motor. In such a conventional device, an ultrasonic motor consists of a stator and a moving body which is in contact with the stator. The moving body is driven by progressive oscillations in the stator, which are produced by a voltage. The frequency of the voltage is sequentially modulated, the drive speed of the moving body at each frequency is measured, the frequency at the highest drive speed is stored, and the ultrasonic motor is driven at the voltage corresponding to the frequency of the highest drive speed. Thus, control is performed so that the drive frequency enters a resonant state with respect to the shape and size of the oscillations, thereby maximizing the drive speed and improving drive efficiency of the ultrasonic motor.
In ultrasonic motors for use in high-speed systems, it is important to have, a quick rise response time when the ultrasonic motor is started. Unfortunately, an ultrasonic motor as disclosed in Japanese Patent Publication No. Sho 59-156168 may have an inadequate rise response time when used in high-speed systems. For example, the ultrasonic motor may require an undesireably long drive time to reach a target drive speed. In addition, when the drive time becomes undesireably long, the demand current product (product of the demand current and the time) becomes undesireably large.
Accordingly, it is an object of the present invention to provide an ultrasonic motor having a reduced rise response time when the ultrasonic motor is started.
It is also an object of the present invention to provide an ultrasonic motor having reduced demand current product.
Objects of the present invention are achieved by providing an ultrasonic motor which is drivable at different frequencies and has a desired target speed, each frequency corresponding to a respective speed and a respective torque. The ultrasonic motor enters a specific mode after being started and driven. The ultrasonic motor comprises a drive control unit which, when the ultrasonic motor is started, drives the ultrasonic motor at a frequency corresponding to approximately a maximum torque. When the ultrasonic motor enters the specific mode, the drive control unit drives the ultrasonic motor at a frequency corresponding to approximately the target speed.
Objects of the present invention are also achieved by providing an ultrasonic motor which is drivable at different drive voltages and has a desired target speed. Each drive voltage having corresponding frequencies, each frequency of each drive voltage corresponding to a respective speed and a respective torque. The ultrasonic motor enters a specific mode after being started and driven. The ultrasonic motor comprises a drive control unit which, when the ultrasonic motor is started, selects a drive voltage, and drives the ultrasonic motor at the selected drive voltage and at a frequency of the selected drive voltage which corresponds to a maximum torque. When the ultrasonic motor enters the specific mode, the drive control unit selects a different drive voltage, and drives the ultrasonic motor at the selected, different drive voltage and at a frequency of the selected, different drive voltage which corresponds to approximately the target speed.
Moreover, objects of the present invention are achieved by providing an ultrasonic motor which is drivable at a first drive voltage and second drive voltage and has a desired target speed. The second drive voltage is lower than the first drive voltage, the first and second drive voltages each having corresponding frequencies. Each frequency of the first and second drive voltages corresponding to a respective speed and a respective torque. The ultrasonic motor enters a first mode after being started and driven, and enters a second mode after entering the first mode. The ultrasonic motor comprises a drive control unit which, when the ultrasonic motor is started, drives the ultrasonic motor at the second drive voltage and at a frequency of the second drive voltage which corresponds to approximately a maximum torque. When the ultrasonic motor enters the first mode, the drive control unit drives the ultrasonic motor at the first drive voltage and at a frequency of the first drive voltage which corresponds to approximately a maximum torque at the first drive voltage. When the ultrasonic motor enters the second mode, the drive control unit drives the ultrasonic motor at the first drive voltage and at a frequency of the first drive voltage which corresponds to approximately the target speed.